When drilling into hard earth, such as rock, it is necessary to produce an impact force at a penetrating end of a drilling tool in order to pierce through the rock. Typically, the drilling tool includes a drill motor energized by pressurized fluid, such as compressed air, to reciprocate a piston against a bit and thus create the necessary impact force to drive the drilling tool deeper into the earth. The drill motor and bit are separate elements joined together by a suitable connection allowing the drill motor to be the driver element and the bit to be driven element. In this manner, axial and torsional forces are transmitted from the drill motor to the bit.
The prior art teaches the use of mating spline connections between the drill motor and the bit to transfer torsional force from the drill motor to the bit and provide for axial movement of the bit relative to the drill motor upon impact of the piston. Conventional splines on bits and drill motors are manufactured so that the load bearing and non-load bearing faces of the splines straddle the radial center lines of both the driver and driven elements. When a rotational force is applied to the driver element, the driver element rotates about its axis until its load bearing spline faces contact the load bearing spline faces of adjacent splines on the driven element. The result is a point contact due to misalignment of the load bearing spline faces. This point contact, coupled with the axial relative movement between the bit and drill motor, causes rapid wear, galling and stress concentrations in the splines. Hence, the common prior art spline connection between a bit and a drill motor is susceptible to premature failure due in part to the shape of the load bearing spline faces.
In order to overcome this point contact due to misalignment of spline faces, the prior art teaches to orient the load bearing faces of both driver and driven splines in radial planes so that upon contact the entire surface of each of the load bearing faces makes contact. As perhaps best shown in Badcock U.S. Pat. No. 3,334,693 issued Aug. 8, 1967, a jack hammer tool is disclosed including a splined coupling having three radially extending load bearing faces on the mating internal and external splines. A uniform radially measured clearance is provided between the internal and external splines, whereby a lateral shift of one of the splines relative to a centered position results in an undesirable contact between the internal and external splines at locations other than the load bearing faces. When this contact at locations other than the load bearing faces is coupled with relative axial movement between the two members, abrasion in the form of galling will result. In the Badcock connection, this undesirable contact can occur at any location between the spline members due to the equal clearance spacing. When galling occurs at or near the roots of the external threads, a reduction in the critical root diameter of the external spline supporting member, e.g., bit shank, results thereby substantially reducing the structural integrity of the damaged member.